Optical disks are widely used in the forms of CDs and DVDs, the development of a next generation optical disk using blue laser has been advancing, and such an optical disk has been required to have a larger capacity. In addition, a super multi-drive, wherein a single optical disk drive has the functions of not only the replay of a CD but also the replay of a CD and a DVD and the record/replay of a CD-R/RW, DVD-RAM and DVD-R/RW, has been commercialized and is considered to be a mainstream product in future.
An optical disk medium has to be retrieved from an optical disk drive and replayed with another drive. To that end, when data is recorded with a single optical disk drive, the processing for judging the quality of the recorded data called verification is applied in many cases in order to confirm that the recorded data can be reproduced with another drive. Data for error correction using a Reed-Solomon code or the like is added to the data recorded in an optical disk medium so that the data is correctly reproduced by applying error correction processing to cope with the defects of the optical disk medium, the degradation of the optical disk drive with age, and the like. In verification processing, it is the common practice to: count the number of error correction when the recorded data is reproduced; judge whether or not there is an appropriate allowance in the correction capability; and thus judge whether or not the quality of the recorded data is good. The appendix M of JIS X6243, one of the DVD-RAM specifications, shows the guideline of the verification with an allowance in the error correction capability and the method called the replacement or alternation processing of a sector when the quality of the recorded data is judged to be insufficient as a result of the verification.
In an optical disk drive and an optical disk medium, the record/replay speed thereof is also increasing simultaneously with the aforementioned larger capacity thereof, and an optical disk drive that can record/replay at a speed 52 times faster in the case of a CD-R and 16 times faster in the case of a DVD±R than a standard speed has also been commercialized.
When the reproduction speed of an optical disk drive is increased, jitter appears and the quality of the reproduced data such as an S/N ratio deteriorates. This is, as generally known, because the influence of the circuit noise represented by amplifier noise increases in accordance with the increase of the speed. Further, as is also commonly known, an optical disk medium, such as a CD-R or a DVD-R, of a write-once type using an organic coloring matter has the same reflectance as a read-only memory type medium such as a CD-ROM or a DVD-ROM and therefore is comparatively hardly affected by amplifier noise. In contrast, a rewrite type medium using a phase-change material has a lower reflectance than the above mediums and therefore is strongly affected by amplifier noise.
A direct slice method and a PRML (Partial Response Maximum Likelihood) method are generally employed as a method for binarizing a regenerative signal of an optical disk. The PRML method is excellent in improving an S/N ratio and hence widely used as the means of securing the higher speed and larger capacity of a magnetic disk. The PRML method is a method that binarizes a regenerative signal to form a most probable bit string while comparing the specific regenerative signal at the time N in the consecutive regenerative signals with a target signal. The Viterbi decoding method, one of the ML methods, is widely used since it can drastically reduce the circuit size. The direct slice method has been used from the past as a method of replaying an optical disk, but it is obvious that the method has limitation on the increase of the speed and capacity. For that reason, the PRML method has been applied also as the reproduction means of an optical disk. However, even the Viterbi decoding method requires a larger circuit size than the direct slice method. In this light, the standardization of CDs and DVDs is based on the direct slice method and most of the previous generation optical disk drives employ the direct slice method.
A rewrite type optical disk is strongly required to have a higher speed. In the case of a DVD-RAM in particular, recorded data can instantly be verified and hence the verification at the same speed as the recording is necessary for securing the performance of an optical disk drive. In an optical disk drive that records and reproduces data at a high speed on such a rewrite type optical disk, it is necessary to employ the PRML method in order to improve the S/N ratio. It is also necessary at the same time to assure the reproducibility of data with a previous generation optical disk drive employing the direct slice method.
As an example of the PRML method for an optical disk, JP-A No. 296987/1999 discloses the technology of adaptively tracking the target signal level of a Viterbi decoder in conformity with a regenerative signal level.
As an example of an optical disk drive employing both the binarizing methods of the PRML method and the direct slice method, JP-A No. 298514/2002 discloses the configuration of an optical disk drive wherein a signal subjected to PR equalization processing is binarized on the basis of the expected level of an edge.
Further, JP-A No. 149679/1999 discloses the technology of processing verification with an optical disk drive employing the PRML method by observing a regenerative signal during the record of data and simultaneously observing both the response from the data recorded at the last minute and the response of the signal reproduced at the same time as the record.